Plasma arc sprayed modified alumina high emittance coatings for noble metals

ABSTRACT

A method of applying a controlled emittance to a noble metal selected from the group including ruthenium, rhodium, palladium, osmium, iridium and platinum, and intra-alloys thereof. The coating is applied by plasma arc spraying modified alumina on a surface of one of the noble metals group, the alumina being modified by having a hydrochloric acid washed alumina powder contain a dispersion of a noble metal black selected from the above group. A cladding of one of the group of noble metals above, having a layer of plasma arc sprayed modified alumina bonded on at least one surface thereof, the alumina being modified as indicated in the above method.

Unite States Patent [1 1 Blumenthal et al.

[ PLASMA ARC SPRAYED MODIFIED ALUMINA HIGH EMITTANCE COATINGS FOR NOBLEMETALS [75'] Inventors: Jack L. Blumenthal, Los Angeles;

David F. Carroll, Torrance; John R. Ogren, La Palma, all of Calif.

[73] Assignee: The United States Atomic Energy Commission, Washington,DC.

22 Filed: Nov. 5, 1970 211 Appl. No.: 87,055

Related US. Application Data [63] Continuation-impart of SenNo.'695,796, Jan. 4,

[56] References Cited UNITED STATES PATENTS 2,946,829 7/1960 Likins etal. 252/466 PT [111 3,751,295 Aug. 7, 1973 2,948,672 8/1960 Brennan 4.252/466 P'l' 2,965,564 12/1960 Kirshenbaum et al. 252/466 PT 3,183,3375/l965 Winzeller et a1. ll7/93.l PF

Primary Examiner-Alfred L. Leavitt Assistant Examiner-J. H. NewsomeAttorney-Roland A. Anderson [57] ABSTRACT A method of applying acontrolled emittance to a noble metal selected from the group includingruthenium, rhodium, palladium, osmium, iridium and platinum, andintra-alloys thereof. The coating is applied by plasma arc sprayingmodified alumina on a surface of one of the noble metals group, thealumina being modifled by having a hydrochloric acid washed aluminapowder contain a dispersion of a noble metal black selected from theabove group.

A cladding of one of the group of noble metals above, having a layer ofplasma arc sprayed modified alumina bonded on at least one surfacethereof, the alumina being modified as indicated in the above method.

1 Claim, 3 Drawing Figures murm H915 I 3.151.295

David F. Carroll Jack L. Blumenthal John R. Ogren I N VEN 'TORS 1 PLASMAARC SPRAYED MODIFIED ALUMINA HIGH EMITTANCE COATINGS FOR NOBLE METALSBACKGROUND OF THE INVENTION This is a continuation-in-part ofapplication Ser. No. 695,796, filed Jan. 4, 1968.

Controlling the surface emittance of radioisotope containment vesselsfor aerospace applications has been a problem in the prior art. Thisproblem has been particularly severe in view of safety requirementswhich are that no release of the radioactive contents be allowed underany conceivable situation, that is, both normal operation and allabnormal modes of operation, including abort modes. The most probablesituation involves exposure of a fuel vessel to terrestial gases, suchas air or water vapor and carbon dioxide, at elevated temperatures of upto 2,500 F for an extended period of time measured in years. Thesurvivability of the vessel in this situation depends strongly upon thesurface temperature of the vessel which can be lowered to an acceptablelevel by the use of a high emittance coating according to the invention.

Containment material for radioisotope vessels should include arefractory metal strength member clad with a noble metal. The refractorymetals usually include vanadium, niobium (columbium), tantalum,chromium, molybdenum, tungsten and rhenium. The refractory metalprovides high temperature (l,600 F) creep strength, and the noble metalprovides protection from the rapid, high temperature oxidation thata'bare refractory metal container would incur. The noble metal claddinghas a disadvantage and this is its low emissivity, about 0.2.Hemispherical total emissivity, E,,,, is the ratio of the radiancy (rateof emission of radiant energy from a unit area of surface) of a polishedsurface to that of a black body at the same temperature.

The steady state temperature of a radioisotope capsule in a ground abortsituation, for example, is determined principally by the emissivity ofits surface; the higher the emissivity, the lower the capsuletemperature. When noble metal claddings are used for oxidationprotection of refractory metals, they degrade at a rate dependentstrongly upon temperature. Thus, any reduction in capsule temperatureachieved by increasing the emissivity of a noble metal cladding willenhance the capsule ground abort survivability.

SUMMARY OF THE INVENTION The invention generally relates to a means toincrease the total emittance of noble metals. It has been surprisinglyfound that when alumina, M is modified with a noble metal black andplasma arc sprayed on a noble metal, as described above, the totalemittance increase over that of the bare noble metal is approximately250 percent. The invention is comprised of a method to modify aluminawith a dispersion of a noble metal black and then to plasma arc spraythe modified alumina on the noble metal to provide a high emittancecoating.

An object of this invention is to provide a method for applying a highemittance coating of alumina modified with a'noble metal black.

Another object of the invention is to provide a method for modifyingalumina with a noble metal black.

Still another object of the invention is to provide a high emittancecladding formed of a noble metal having a modified alumina coating.

A further object of the invention is to provide a noble metal coatingwhich retains its ductility after high temperature service in both vacuaand air.

A still further object of the invention is to provide a modifiedalumina, high emittance coating for noble metal substrates in which nochemical interactions occur, and the inherent oxidation resistance ofthe noble metal substrate remains unaffected by the coating.

Further objects and advantages of the invention may be brought out inthe folllowing part of the specification wherein small details have beendescribed for the competence of disclosure, without intending to limitthe scope of the invention which is set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS Referring to the accompanyingdrawings, which are for illustrative purposes:

FIG. 1 is an isometric view of a noble metal substrate having a plasmaarc sprayed modified alumina high emittance coating thereon;

FIG. 2 is a magnified end view of a substrate of a platinum-rhodiumalloy having a plasma arc sprayed platinized alumina coating bonded ontwo opposite surfaces thereof; and

FIG. 3 is an illustration of a noble metal cladding for a refractorymetal and having-a plasma arc sprayed modified alumina on inner andouter cylindrical'surfaces of the cladding in accordance with theinvention, the end view thicknesses of the cladding and coatings beingsubstantially exaggerated.

DESCRIPTION OF THE PREFERRED EMBODIMENT The inventive process iscomprised of modifying alumina with a noble metal black selected fromthe noble metal group including ruthenium, rhodium, palladium, osmium,iridium and platinum and intra-alloys thereof. The alumina may bemodified with a noble metal black in all concentrations such that asubstantial quantity of the noble metal remains as a finely dispersednoble metal black after plasma arc spraying the modified alumina on thenoble metal substrate. In other words, as long as the noble metalremains in a fine dispersion as a black, it is not in excess in themixture with the alumina.

For the purpose of the process, the source of the noble metal may be ina soluble form so that it may be put into solution with the alumina.Such noble metal compounds are: chloroplatinic acid, IrCl [Rh (NI-l]Cl,, RuCl .5I-I,O, and osCl An example of the process to synthesize afive weight percent platinum modified (platinized) alumina is asfollows:

1. Approximately 147.5 grams of activated alumina, Al,0,, are added to abeaker of concentrated hydrochloric acid.

2. The concentrated hydrochloric acid and the alumina are boiled forapproximately two hours with intermittent stirring.

3. The hot concentrated hydrochloric acid is removed from the alumina,and the alumina is washed with several aliquot portions of distilledwater until approximately 5 gallons of water is used.

4. The alumina is removed from the water and dried in air attemperatures ranging from room temperature to 250 F.

5. Approximately 22.58 grams of chloroplatinic acid, H Pt Cl 6H O isdissolved in 250 ml of water, and the dried alumina is added thereto.

6. The chloroplatinic acid and the alumina are blended for 12 hours to18 hours at room temperature.

7. The temperature is then raised to 90 to 95 C. and the blending iscontinued for 3 to 4 hours.

8. The alumina is removed and dried in a vacuum oven for 48 to 65 hoursat 60 to 65 C.

9. The resulting dried cake of platinized alumina is pulverized and isheated for 3 to 4 hours in flow hydrogen at from 500' to 700 C to removethe hydrogen chloride.

The last step may also be accomplished in still air at 500 C.

The noble metal or intra-noble metal alloy substrate is then plasma arcsprayed with the platinized alumina to establish a high emittancecoating having a thickness of from 0.003 inch to 0.009 inch. Thethickess of the noble metal or alloy used for cladding on radioisotopecontainmentvessels is in the range of from 0.020 inch to 0.100 inch.There is shown in FIG. 1 a substrate in the form of a flat plate of oneof the noble metals stated, having diffusion bonded on one side 11, amodified alumina high emittance coating 12, plasma arc sprayed thereon.The diffusion bonding is shown in the irregular surfaces as at 13 wherethe modified alumina is joined to the noble metal or intra-alloythereof. This can be generally seen only through high magnification.

In FIG. 2 there is shown a substrate 17 ofa platinum- 10 weight percentrhodium alloy, having diffusion bonded to opposite sides, a thin layerof plasma arc sprayed platinized alumina 18. The excellent diffusionbonding at 19 between the coating and the noble metal substrate isindicated by the irregular edge penetration of the coating into thenoble metal. The drawing in FIG. 2 was made in exact size from a copy ofa 200x phtomicrograph. The coating 18 has a dark gray color interspersedwith totally black shadows formed in recesses 20 in the coatings.

The pretreatment acid wash described above produces a significantimprovement in the emittance of the platinized alumina over aluminawhich has not been subjected to the pretreatment. Platinized aluminawhich has been pretreated with the hot hydrochloric acid wash exhibitsan emittance in the general range of from about 0.7 to about 0.8,whereas the emittance of the untreated platinized alumina may range fromabout 0.2 to about 0.7.

In FIG. 3, there is shown a tube 23, having external and internalmodified coatings 24 and 25, respectively, formed upon a noble metal 26,such as that shown in FIGS. 1 and 2. The thickness of the coatings andthe noble metals are exaggerated but are shown to illustrate a tubularcladding for use on a refractory metal for containing radioisotopes.

The invention and its attendant advantages will be understood from theforegoing description, and it will be apparent that various changes maybe made in the form, construction and arrangement of the parts of theinvention without departing from the spirit and scope thereof orsacrificing its material advantages, the arrangement hereinbeforedescribed as being merely by way of example. We do not wish to berestricted to the specific forms shown or uses mentioned, except asdefined by the accompanying claims, wherein various portions have beenseparated for clarity of reading and not for emphasis.

We claim:

1. A method of making a high emittance oxidation resistant claddingcomprising:

a. boiling alumina particles having a size range of from 44-74 micronsin concentrated hydrochloric acid;

b. washing said alumina particles in water;

0. removing and drying said alumina particles in air;

d. blending said alumina particles into a dissolved soluble compound ofone or more of the noble metals selected from the group consisting ofruthenium, rhodium, palladium, osmium, iridium, and platinum andintra-alloys thereof at room temperature; heating the solution toapproximately C to drying in vacuum at temperatures from approximately65 C; pulverizing the formed dried cake of mixed metal and alumina;heating the dried mixture in a gas selected from the group consisting ofair and hydrogen for a period of approximately 3 to 4 hours attemperatures from approximately 500 C to 700 C; and i. applying saiddried mixture to a substrate of noble metal.

